I've created a small piece of software that allows me to invest into CFDs and gain some small profits. However, most brokers I've found require high opening/closing fees that are in most cases a way larger than my earnings (It does rely on small movements and only holds it for a few minutes). Is anyone of you in aware of a broker that has very small opening / closing fees or, even better, that only charges for my profits?
that only charges for my profits - do you understand how CFDs work? For the sake of simplicity, there are four aspects to CFD trading:
(1) bid/offer spread - differs on whether you're doing OTC contract, or have DMA access
(2) margin requirements - usually tiered and each tier has % of required margin
(3) overnight funding
(4) commissions - there are CFDs with no commissions (e.g. cash indices, futures); and there are CFDs where commissions are calculated as % (e.g. equity CFDs).
So, to dissect your question:
invest into CFDs - CFD is a contract, you need to be specific - what is the underlying asset?
high opening/closing fees - depends on asset class, see above
only holds it for a few minutes - you're trying to scalp the market. Some brokers may not like that.
only charges for my profits - no, that's not how CFD operate.